Aviation gasoline (aviation fuel) generally contains an aviation alkylate base fuel and a lead-based additive package. A conventional aviation fuel formulation contains light alkylate, toluene, C4 to C5 paraffins and tetraethyl lead. Current formulations comprise 75-92 vol. % light alkylate, 5-18 vol. % toluene, 3-20 vol. % C4 to C5 paraffins and 2-4 ml/gallon tetraethyl lead (TEL). The industry standard Grade 100 aviation gasoline contains up to 4 ml of TEL/gallon of fuel while Grade 100LL (low lead) aviation gasoline contains up to 2 ml TEL/gallon of fuel. Tetraethyl lead is conventionally added as an octane booster to improve the anti-knock properties of the aviation fuel over the anti-knock properties of the aviation alkylate base fuel. Specifications for aviation gasoline are detailed in ASTM D910-07a. Grade 100 aviation gasoline and Grade 100LL aviation gasoline are two grades of aviation gasoline having properties described by the specification.
The use of tetraethyl lead in fuels, particularly in automotive gasolines, has been restricted for many years due, in part, to health and environmental concerns as well as catalyst poisoning effects in automobile catalytic converters. Aviation gasolines have been allowed to contain tetraethyl lead since no suitable substitute has been found with adequate knock resistance to allow the current fleet of aircraft engines to operate properly. Current U.S. regulations set a maximum amount of tetraethyl lead in aviation fuels at 4.0 ml TEL/gallon. The continued use of tetraethyl lead nonetheless remains an environmental and health concern which has not been completely resolved. The possibility of further restrictions, or a prohibition, on the use of tetraethyl lead in aviation gasolines therefore exists.
Alternatives to the use of tetraethyl lead are known. For example, methylcyclopentadienyl manganese tricarbonyl (MMT) has been used as an antiknock agent in motor fuels since around 1975, first as a supplement to leaded agents, and then as a replacement to produce lead-free gasoline. However, questions have also been raised concerning the production of undesirable emissions using MMT.
One possible option is to hydrogenate di-isobutylene to form a mixture of isoparaffins, predominately 2,2,4-trimethylpentane or “iso-octane.” Iso-octane derived from such a process may then be used to form a suitable aviation gasoline composition.
Aromatic amines and alkyl ethers have been proposed as substitutes for tetraethyl lead. These also have been found to have environmental and performance limitations as aviation gasoline additives.
In view of the current limitations placed on the use of tetraethyl lead it is desirable to produce aviation fuel compositions which contain reduced levels of lead, or do not require the presence of lead-based additives.